This proposal is an anatomical study of the responses of individual axons of retinal ganglion cells to different environmental conditions as they regenerate after damage to the optic nerve. Two species of fish, the rainbow cichlid and the bluegill sunfish, will be used. The project is divided into 3 parts: 1) a study of the pattern and timing of regeneration onto a recrom which is devoid fo residual retinal axonal material 2) a study of the effects of electrical activity on the branching of regenerating axons both as they make their ways through the damaged region of the nerve and as they make new terminal arbors on the tectum; and 3) a study of the effects of partial retinal ablation on the tectum. For the first study one optic nerve in each fish will be crushed and kept off the tectum until the debris from the degenerating axons has disappeared. Then the optic nerve will be allowed to regenerate onto the tectum, and the time and precision of reestablishment of a retinotopic map on the tectum will be determined by filling small groups of regenerated axons with cobaltouslysine to visualize them both in sections of the optic nerve and tract and on the whole flat-mounted tectum. The positions of these filled axons will be compared both with respect to the time course of regeneration and to the precision of the normal map. For the second study the optic nerve will be crushed and then allowed to regenerate either in the absence of electrical activity (by injecting an actionpotential blocker into the eye) or in the presence of extra electrical activity (caused by stimulating the nerve externally). Again the regenerated axons will be visualized by filling small groups of them with cobaltous-lysine; the number of branches per axon will be quantified both as the axons cross the damaged region of the nerve and as they terminate in the tectum. The number of branches at these points will be compared with the precision of reestablishment of the retinotopic map to determine whether axons regenerate more precisely with or without electrical activity. The final study is an attempt to determine whether loss of a portion of retinal afferents leads to loss of tectal cells in adult fish. Cells at the tectal margins will be labeled with 3Hthymidine before partial retinal ablation; later the tecta will be sectioned and examined radioautographically to determine whether the labeled cells have disappeared, indicating actual loss of neurons. These studies combine to define some of the parameters which affect successful regeneration in the central nervous system.